The present invention relates to a driving circuit for a stepping motor used as a paper feed driving source mounted on, for example, a line printer, and particularly to a stepping motor torque control system.
Generally, the stepping motor is driven by a driving signal consisting of excitation pulses to execute stepping rotation. When the capacity of the power supply provided for a piece of equipment having a stepping motor is relatively small, the stepping motor driving voltage frequently fluctuates depending on the operating conditions or operating environment. Moreover, the speed of the stepping motor is frequently controlled in accordance with the equipment operating conditions. When the driving voltage fluctuates upward and the motor speed decreases, the height and width of the excitation pulse increase and excessive current flows through the motor coil. Resultingly, the motor torque may decrease. Moreover, the stepping motor may become non-operational due to long periods of continuous operation resulting in overheating. In addition, the mechanical resonance point may be deviated to the relatively low speed side depending on the stepping motor. In this case, if the motor is rotated at a low speed by a high driving voltage, a large torque is generated and vibrations and noises due to resonance may be produced.
To prevent the troubling effects caused by the above-mentioned excessive power, a system is proposed to effectively decrease the driving voltage by applying intermittent chopping to the excitation pulse. The intermittent chopping controls the effective driving voltage without affecting the rotational characteristics of the stepping motor by changing the non-excitation time in the excitation time at a very high speed while the excitation pulse is outputted.
In the conventional driving system, intermittent chopping is unconditionally applied to excitation pulses regardless of variation of the driving voltage level and change in the motor speed. In this case, a problem rarely occurs when the driving voltage level is high and the motor operates at a high speed. However, if the driving voltage level lowers and the motor is decelerated, the effective driving voltage remarkably decreases and the torque of, the stepping motor decreases.